Although the present invention is applicable to any compressors, the present invention and the problem on which it is based will be described in detail in relation to a rotary compressor.
A compressor is usually driven by a transducer in a directly coupled manner or via a gearing and compresses a medium, in particular a gas, preferably air, as soon as the compressor is set in movement. However, this mode of operation has some drawbacks with regard to establishing a compressed air network. In a modern industrial plant, compressed air is used for many different applications. Depending on the number of loads connected to the compressed air network, the compressed air requirement increases or decreases. In order to maintain the required pressure level, the compressor would therefore switch continuously between full load and standstill. Control of this type would have a negative effect on the service life of all driven and driving components. Added to this is the increased power consumption in the start phase, which has an equally negative effect on the operating costs. In order to counteract this, various types of control are used in compressors. The function thereof is to minimise the energy consumption and wear and to maximise the availability. A control unit in what is known as a screw-type compressor can be what is known as a gas inlet valve. This is connected upstream of the compressor block of the compressor. The gas inlet valve is intended to allow a non-return function, in which backflow of gas and/or fluid from the compressor into the environment of the compressor is prevented, no-load control and full-load control and/or proportional control.
In the case of no-load control of the compressor, the gas inlet valve may allow only a particular amount of air or process gas into the compressor. This measure can be explained as follows. The start-up of the compressor involves an increased energy requirement as measured at full-load operation owing to the mass inertia of the moved components. In order to minimise these peak loads, it helps to minimise the work which is carried out during compression of the medium, by reducing the incoming air flow to a minimum amount. For this purpose, the gas inlet valve comprises a spring-biased valve body. For switching from no-load operation to full-load operation, this body is lifted from the valve seat, against the spring force of a spring which presses the valve body against its valve seat, by means of a pressurised piston. A gas inlet valve of this type comprising a spring-biased valve body is disclosed, for example, in DE 602 10 088 T2 or in U.S. Pat. No. 6,431,210 B1. However, this construction of the gas inlet valve has proven disadvantageous in that the valve spring, which presses the valve body against the valve seat when the compressor is in the no-load operating state, must be very powerfully dimensioned and thus requires particular care during disassembly and assembly. Owing to the high spring bias, work on the valve spring involves an extremely high risk of injury.
In order to avoid this, DE 603 07 662 T2, for example, proposes a gas inlet valve in which a large valve spring of this type can be dispensed with. For this purpose, a double piston which can be pressurised on two sides is exposed, on the side remote from the gas outlet of the gas inlet valve, to the negative pressure applied to the gas outlet of the gas inlet valve and, on the opposite side, to a control pressure, in order to press the valve body against its valve seat. However, this requires a complex, finely adjusted and thus fault-prone pneumatic system in order to ensure reliable operation of the gas inlet valve.
DE 689 04 263 T2 discloses a screw-type vacuum pump which contains a pump housing comprising suction and pressure openings on the opposing sides thereof; interlocking outer and inner rotors which contain means for pumping a gas from the suction opening when the rotors are rotated; a force transmission means for rotating the rotors, which contains a gear box comprising an oil tank; oil circulation means, which contain an oil pump and an oil cooler, for circulating lubricating oil to the pump housing to lubricate the rotors; and a stop valve which communicates with the suction opening and comprises a valve housing which defines a gas flow space and a cylinder space, the gas flow space being isolated from the cylinder space in terms of flow and comprising a valve seat, and a valve body which is normally pressed in the closed position in order to close the valve seat and close the suction opening, the valve body including a piston which is fitted into the cylinder space in order to divide the cylinder space into an oil chamber and an air chamber at atmospheric pressure, a three-way changeover valve being provided which selectively connects the oil chamber to the oil tank via a relief path when the vacuum pump is stopped and to the oil circulation means in a position downstream of the oil pump when the vacuum pump is in operation.
DE 603 07 662 T2 discloses a compressor containing a compressor element which is provided with a rotor chamber, to which an inlet pipe and an outlet pipe are connected, a tank in the outlet pipe and a pressure control system which comprises an inlet valve positioned in the inlet pipe, a piston which is connected to the inlet pipe and can be moved in a cylinder, a bypass which bypasses said inlet valve and in which are positioned in succession, between the inlet pipe and the rotor chamber, a gas flow limiter and a non-return valve which allows only gas into the rotor chamber, and a gas pipe which connects the tank to the part of the bypass which is located between the gas flow limiter and the non-return valve, and a relief valve positioned in said gas line.
DE 602 10 088 T2 discloses a volumetric compressor which comprises a compressor element comprising a compression space to which an inlet line, which can be closed by means of an inlet valve, and a pressure line, in which a pressure vessel is installed, are connected, the inlet valve comprising a valve element which cooperates with a valve seat, said element being connected to a piston which can be displaced in a hollow space in a cylinder-forming housing, and a resilient element which presses this valve element towards the valve seat, while a control line connects the interior of the pressure vessel to a cylinder chamber formed between the operative side of the piston and the housing.
U.S. Pat. No. 6,431,210 B1 discloses an inlet valve for a gas compressor, the inlet valve comprising a piston which can be moved in a housing chamber and can be moved towards and away from a housing inlet. A valve disc can be moved by means of the piston, the valve disc comprising an opening for controlling an air flow from the housing inlet into the housing chamber. The inlet valve further comprises a flexible component which cooperates with the valve disc to close the opening.
GB 385 801 A discloses an arrangement for automatically starting up a compressor, in particular for starting up under no load. During start-up of the compressor, a start valve keeps the suction line closed. The start valve is held in its closed operating state by the pressure of a spring. The valve opens when the compressor is in operation, since the resulting negative pressure acts directly on the valve.